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Characteristics of NdFeB Magnets

This section provides information of the physical properties of Neodymium Iron Boron (NdFeB) magnets.

As already discussed in previous sections, the NdFeB magnet has various grades:- each grade has its own magnetic properties (relating to strength of magnetic field output and resistance to demagnetisation, maximum recommended operating temperature and temperature coefficients).

The grades have other physical properties which are similar between the grades. Below is an overview of these properties:-

Summary of Physical Properties of Neodymium Iron Boron, NdFeB, magnets

Characteristic

Symbol

Unit

Value

Density

D

g/cc

7.5

Vickers Hardness

Hv

D.P.N

570

Compression Strength

C.S

N/mm2

780

Coefficient of Thermal Expansion

C//

10-6/°C

3.4

 

C^

10-6/°C

-4.8

Electrical Resistivity

r

m Ω.cm

150

Temperature Coefficient of Resistivity

a

10-4/°C

2

Electrical Conductivity

s

106S/m

0.667

Thermal Conductivity

 k

kCal/(m.h.°C)

7.7

Specific Heat Capacity

 c

kCal/(kg.°C)

0.12

Tensile Strength

σUTS, or SU 

kg/mm2

8

Young's Modulus

 l / E

1011N/m2

1.6

Flexural Strength

 b

10-12m2/N 

9.8

Compressibility

s

10-12m2/N 

9.8

Rigidity

 E.I

N/m2

0.64

Poisson's Ratio

 

0.24

Curie Temperature

Tc

°C 

310

Structural use of Neodymium Iron Boron, NdFeB, magnets
There is a risk of chipping or breaking the magnets because all magnets are inherently brittle. The Neo magnets are less brittle than SmCo. It is advised to not put magnets in conditions of mechanical stress e.g. in load bearing situations.

The Effects of Radiation on Neodymium Iron Boron, NdFeB, magnets
The NdFeB magnets may be demagnetised by radiation. The Neodymium Rare Earth magnets do not perform as well as SmCo Rare Earth magnets. E.W. Blackmore, (TRIUMF, 1985) and A.F. Zeller & J.A. Nolen (National Superconducting Cyclotron Laboratory, 09/87) demonstrated SmCo having a better performance, with Sm2Co17 offering 2-40 times better radiation resistance than NdFeB. Some NdFeB grade are demagnetised to half their maximum performance with a proton beam radiation of 4 x 106 rads and are completely demagnetised with a proton beam radiation of 7 x 107 rads. A rule of thumb is to select magnets with higher Hci values, designed to operate at high Pci and, where possible, to have radiation shielding protecting them when being subjected to any levels of radiation. The user of the magnets would need to test for effectiveness of the magnets as the magnet suppliers do not have the equipment to test for suitability of magnet grades for environments with raised levels of radiation.

Neodymium Iron Boron, NdFeB, magnets and corrosion resistance
The NdFeB magnets require a protective coating / surface finish to minimize the effects of corrosion. Iron within the structure can ‘rust’ which causes a permanent structural change in NdFeB which results in a permanent weakening of the magnetic performance – the worst case scenario is a total loss of magnetism.

A NdFeB magnet kept in dry conditions will not corrode and will retain its performance theoretically for ever (if not subjected to excessive heat, radiation or strong external magnetic fields). If the conditions are wet, it is recommended that alternative magnets be considered for use of that the magnet design try to protect the magnet from moisture (e.g. encasing, modified coatings such as zinc plus rubber, etc). The plating / surface finish should be hermetic for best corrosion protection – scratched or damaged surfaced may render the affected region more prone to corrosion. Marine environments (salt sprays, sea water) are particularly corrosive and far from ideal for NdFeB. In critical applications where corrosion and magnet failure are unacceptable, magnets such as ferrite or SmCo may be more suitable. Please note that any claims that a NdFeB magnet will not corrode is misleading. It is claimed that higher Hci magnets resist corrosion better although the empirical results are not so conclusive (a trend suggesting an improvement in corrosion resistance exists but it is not guaranteed). It is the application and the overall design that determines how well the magnet will perform in damp environments.

Table comparing main coating types

COATING APPLIED

NICKEL

EPOXY RESIN

Ni + EPOXY

Electroless

Powder Spray
Coating

E-Coating

Nickel plating
+ Epoxy E-Coating

Coating Thickness

Range (microns)

12 to 25

25 to 40

20 to 40

15 to 25

25 to 40

Homogeneity

Excellent

Good

Poor

Excellent

Good

Effectiveness versus Magnet Size

Small (<20 grams)

Excellent

Good

Fair

Good

Good

Large (>20 grams)

Fair to Good

Good

Fair

Good

Good

Hours before coating is likely to fail

Temp. & Humidity
(60ºC, 95%RH)

>2500

>500

>1500

>2500

Temp. & Humidity
(85ºC, 85%RH)

>500

>100

>300

>500

Salt Spray
(35ºC, 5% NaCl)

>48

<24

>100

>200

Coating Colour

Silver

Silver

Black

Black

Black

Heat Cycle

Fair

Fair

Fair

Fair

Fair

Heat Resistance

Poor

Poor

Poor

Poor

Poor

Collision Test

Fair

Fair

Fair

Fair

Fair

Film to material adhesion test

Fair

Fair

Fair

Fair

Fair

Glue adhesion test

Fair

Fair

Fair

Fair

Fair

Tolerance accuracy

Excellent

Excellent

Fair

Fair

Fair to Poor

Additional Remarks

15-30 microns Ni-Cu-Ni Standard coating

Epoxy resins are not hermetic

Thickness buildup can be a problem

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